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1680s, from French trillion (late 15c.), from tri- (see tri-) + (m)illion. Cf. billion. In the U.S., the fourth power of a thousand (one thousand billion, 1 followed by 12 zeroes); in Great Britain, the third power of a million (one million billion, 1 followed by 18 zeroes), which is the original sense.
In Britain, France, and Germany, 10^18 or a million cubed.
In the USA and Canada, 10^12.